1. Field of the Invention
The present invention relates to an image formation apparatus employing the electrostatic recording method, such as a plain paper copier (PPC) and an electrostatic printer, and in particular to a wet-type image formation apparatus in which a latent electrostatic image is developed with a liquid-type developer.
2. Discussion of Background
Liquid developers for use in a wet-type image formation apparatus are conventionally disclosed in Japanese Laid-Open Patent Applications 52-17419, 61-180248 and 63-4249.
In general, a liquid developer prepared by dispersing toner particles comprising a coloring agent and a binder resin in a petroleum aliphatic hydrocarbon solvent serving as a carrier liquid is conventionally used in a wet-type plain paper copier (PPC) to obtain satisfactory image-fixing performance. Commercially available petroleum aliphatic hydrocarbon solvents, such as "Isopar C", "Isopar E", "Isopar G", "Isopar H", "Isopar K", "Isopar L", "Isopar M" and "Solvesso 150" (Trademark), made by Exxon Chemical Japan Ltd.; and "Shellsol 71" (Trademark), made by Shell Kagaku K.K., are well known. In addition to the above, cyclohexane, n-hexane, n-heptane, n-octane, isooctane, isododecane and ligroin are used as the carrier liquids for the liquid developer.
The carrier liquids for the liquid developer are required to have the following properties: (1) a high electrical resistivity of 10.sup.14 .OMEGA.cm or more; (2) a low dielectric constant of 3 or less; (3) a low solubility; (4) no odor; (5) transparent; (6) not irritating to the skin; (7) nontoxic; (8) low flammability of 50.degree. C. or more; (9) thermally stable; and (10) quick-drying.
Table 1 shows the boiling point and the average molecular weight of the above-mentioned petroleum aliphatic hydrocarbon solvents, commercially available under the name of "Isopar" from Exxon Chemical Japan Ltd.
TABLE 1 ______________________________________ Boiling Point(.degree. C.) Average Molecular Weight ______________________________________ Isopar C 97.8 to 102.8 -- Isopar E 115.6 to 141.1 120 Isopar G 159.0 to 176.7 149 Isopar H 176.7 to 183.3 154 Isopar K 176.7 to 195.0 156 Isopar L 188.9 to 207.8 165 Isopar M 206.1 to 246.7 177 ______________________________________
The liquid developer can be prepared by kneading a coloring agent, a binder resin and a small amount of additives together with the above-mentioned carrier liquid in a ball mill or Keddy mill to obtain a concentrated toner, and dispersing the concentrated toner in an appropriate amount of the carrier liquid.
Recently, as the plain paper copier (PPC) becomes prevailing in the field of wet-type copying operation, a demand for excellent copy images with high image density is increasing. To satisfy this demand, a large quantity of toner is caused to be deposited on a transfer sheet. However, in the case where the petroleum aliphatic hydrocarbons are used as the carrier liquid for the liquid developer in the plain paper copier, the vapor of the petroleum aliphatic hydrocarbons eventually builds up in the room where the copier is in operation when the amount of toner to be deposited on the transfer sheet is increased. As the concentration of the vapor is increased, the vaporized carrier liquid gives out an offensive or unpleasant odor in the image-fixing performance. In the case where the copier is operated in a room without ventilation, it is necessary to restrict the copying speed, with the offensive odor caused by the vaporized petroleum aliphatic hydrocarbon taken into consideration. When the petroleum aliphatic hydrocarbon is applied to the high-speed copying apparatus employing the wet-type development system, the offensive odor is unfavorably generated in the image-fixing operation. For this reason, the petroleum aliphatic hydrocarbons are not adequate for the high-speed copying operation.
The wet-type plain paper copier is constructed in such a fashion that the liquid developer circulates therein by means of a pump. More specifically, the developer which has not been used for developing a latent electrostatic image is recovered in a cleaning unit by the aid of a cleaning foam roller, and returned to a liquid developer reservoir through a cleaning developer recovery hole. The toner particles in the liquid developer are repeatedly used in the above-mentioned manner, so that toner particles are not wasted, and a large number of copies can be made by a relatively small amount of the liquid developer.
The conventional wet-type plain paper copier has the shortcoming that the image quality is degraded when the copying operation is performed for an extended period of time or the copying operation is resumed after a long non-use period. For instance, abnormal images such as white narrow stripes appear in solid image areas as shown in FIG. 7. This is ascribed to the inadequate circulation of the liquid developer in the copier. The toner particles remaining on the photoconductor after image transfer are collected in the cleaning unit, and the toner particles deposited to the development rollers are scraped off the rollers by scrapers and collected in the development unit, so that most of the toner particles are returned to a liquid developer reservoir by the circulating mechanism and uniformly dispersed again in the carrier liquid to form a liquid-type developer with a predetermined concentration. When the petroleum aliphatic hydrocarbon is used alone as the carrier liquid for the liquid developer, part of the toner particles are left alone in the cleaning unit and the development unit because the above-mentioned petroleum aliphatic hydrocarbon easily evaporates. As the petroleum aliphatic hydrocarbon continues to evaporate for an extended period of time, only the toner particles accumulate just like sludge. These sludgy toner particles are usually washed away by the flow of a circulating liquid developer for cleaning, thereby returning to the liquid developer reservoir. However, the sludgy toner particles are deposited over an extended period of time in a dead space where the flow of the circulating liquid developer for cleaning is weak. In the case where the copying operation ceases for a long time, the water component of the sludgy toner particles gradually evaporates, and finally toner particles are agglomerated in the form of a flake, so that they cannot easily be dispersed again in the carrier liquid. The diameter of the toner particle thus formed in a flake is as large as 1 to 500 .mu.m, when compared with a normal toner particle having a diameter of 0.5 to 0.8 .mu.m, so that these toner particles in the form of a flake, which do not contribute to the development operation, are caught in the gap between development rollers and the photoconductor, which is designed to have a distance of 100 to 150 .mu.m. This causes abnormal images, such as white narrow stripes on a solid formed on the transfer sheet.
In the wet-type image formation apparatus, not only copied images with high image density, but also high-speed copying operation free from the problem of the offensive odor is highly desired. Thus, a silicone oil with a siloxane structure is proposed as the carrier liquid for the liquid developer because it does not generate the unpleasant odor while in use for image fixing.
The above-mentioned silicone oils with a siloxane structure, represented by methylphenyl polysiloxane, have a relatively high viscosity, so that they do not easily evaporate. Therefore, when the silicone oil with a siloxane structure is used as the carrier liquid for the liquid developer, toner particles are prevented from being left alone and accumulating to form hard dried large-size toner particles in the development unit and the cleaning unit in the wet-type image formation apparatus for an extended period of time. As a result, the occurrence of abnormal images, such as white narrow stripes in a black solid, can be prevented.
However, such silicone oils with a siloxane structure are relatively poor in image fixing performance. To improve the image-fixing performance thereof, it is necessary to elevate the temperature of a heat-application roller in an image fixing unit of the copying apparatus.
To elevate the temperature of the heat-application roller in the image fixing unit, however, a considerable amount of the electrical power is consumed. This will inconveniently restrict the attachment of various peripheral devices to the copier because the electrical capacity is limited in the space where the copier is located.
In addition to the above, the cost of the silicone oil with a siloxane structure is generally high, so that the total cost is considerably increased when the above-mentioned silicone oil is used alone as the carrier liquid for the liquid developer.
Furthermore, when the silicone oil with a siloxane structure is used alone as a carrier liquid for the liquid developer, the circulating means for the liquid developer tends to be clogged because of its relatively high viscosity.